1. Field of the Invention
This invention relates to a radiation image read-out apparatus which scans an image recording sheet such as a stimulable phosphor sheet on which radiation image information relating to an object is recorded to thereby obtain an image signal representing the radiation image and carries out image processing of the image signal.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image to obtain an image signal, carrying out appropriate image processing of the image signal, and then reproducing and recording a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is image-processed and then used when the X-ray image is reproduced as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality and exhibiting such characteristics as high contrast, high sharpness, excellent graininess property and the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-104645 and 55(1980)-116340, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored thereon, it is possible to obtain an image which is unaffected by variations in the amount of exposure of the stimulable phosphor to the radiation, even when the amount of exposure varies greatly depending on the imaging conditions, by reading out the emitted light with a photoelectric conversion means at an appropriate read-out gain and converting it to an electric image signal to reproduce a visible image on a recording medium such as photographic film or on a display device such as a CRT.
The read-out condition used at the time of obtaining an image signal and/or the image processing condition used at the time of carrying out image processing with respect to the image signal are specified as items of a menu compiled by statistically analyzing and classifying a large number of radiation images. The classification is based on, for example, what portion of an object is represented by the recorded image (e.g. the head, the neck, the chest or the like in the case where the object is a human body) and what recording mode was used when the image was recorded (e.g. ordinary image recording mode, contrasted image mode, tomographic image mode or the like). Further subclassification, resulting in further menu items, is carried out for specific combinations of the portion of the object and the imaging condition. For example, ordinary image recording of the chest is subclassified into ordinary image recording of the thoracic vertebrae (frontal), the ribs, the clavicle, the scapula and the like.
The need for providing a menu including separate items for the respective read-out conditions and/or image processing conditions will be explained with respect to a specific example.
One condition that should be considered in setting the read-out condition and/or image processing condition is that regarding the treatment of unnecessary portions recorded on the image recording sheet at the time of image recording. Such unnecessary portions occur, for example, at parts of the image recording sheet which are exposed only to scattered radiation or to radiation which directly impinges on the image recording sheet without being transmitted or reflected by the object or which are recorded with images of portions of the object which are not required to be observed. Generally, the read-out condition and/or image processing condition are set to distinguish between the image signals corresponding to the radiation image required to be reproduced and the image signals corresponding to the unnecessary portions.
FIG. 3 is a graph showing a histogram of the quantity L of light emitted during read-out of an image of the chest of a human being in a system employing a stimulable phosphor sheet.
In this figure, the light quantity L emitted by the stimulable phosphor sheet when it is scanned with stimulating light is plotted on the horizontal axis, which has a logarithmic scale, and the frequency of occurrence of emitted light quantity values are plotted on the upper portion of the vertical axis. The image signal S.sub.Q obtained by photoelectric conversion of the light emitted by the stimulable phosphor sheet is plotted on the lower portion of the vertical axis, which has a logarithmic scale.
As shown, the histogram can be divided into 5 regions A-E which correspond respectively to the mediastinum, the heart, the lungs, the skin and soft parts, and regions outside the object.
Where a visible output image encompassing the whole of the mediastinum (region A), the heart (region B) and the lungs (region C) is desired, the read-out condition is set so that the image information falling between L.sub.min and L.sub.1 is appropriately reproduced. More specifically, the read-out condition is set such that L.sub.min and L.sub.1 within the range of light quantities L will respectively give rise to the minimum image signal Q.sub.min and the maximum image signal Q.sub.max corresponding to the minimum density D.sub.min and the maximum density D.sub.max within the range of densities appropriate for the output visible image. What this means is that the read-out condition falls along the straight line G.sub.1 shown in the figure.
On the other hand, if the aforesaid read-out condition (corresponding to the G.sub.1) should be used in a case where it is desired to observe only the heart, the image information corresponding to the heart would be converted to an image signal S.sub.Q covering only the range between Q.sub.1 and Q.sub.2. As this is narrower than would be the case if the image information corresponding to the heart only should be converted to an image signal covering the maximum range between Q.sub.min and Q.sub.max (corresponding to the line G.sub.2), the density resolution of the output visible image will be lower in proportion to the difference. Therefore, where it is desired to observe only the heart (region B), the final read-out condition is set to corresponding to the line G.sub.2 so as to ensure read-out of the image information corresponding to the heart with maximum resolution.
For reasons that will be apparent from the example just explained, read-out conditions and/or image processing conditions are defined in correspondence to a large number of image-recording menu items. The problem thus arises as of how to ensure that the menu item required for a particular read-out and image processing operation can be input with good operational efficiency and freedom from input error.